The main aim of the research project is an understanding of the chemical reactivity of the flavin coenzymes and how this reactivity is used in biological systems. The methods of approach will be the study of representative flavoprotein enzymes and model system studies. Considerable use will be made of steady state kinetics coupled with rapid reaction kinetic studies, and of the technique of replacing the natural coenzyme of certain flavoenzymes with modified flavins, in order to test hypothetical schemes. The basic questions which we are attempting to answer are: 1) What are the steps involved in the reaction of reduced flavins with O2, what species of "activated" oxygen are produced and how do they participate in different enzyme reactions? 2) What are the steps involved in flavin-catalyzed dehydrogenation reaction? Are carbanion intermediates involved as well as flavin-substrate covalent adducts? 3) What factors are involved in formation of charge-transfer complexes involving flavins and what catalytic importance do such complexes have? 4) What are the electron-transfer interactions between flavin and other redox active centers in complex enzymes such as xanthine oxidase? 5) Are deazaflavins better models for pyridine nucleotides than for flavins? In these studies we shall also attempt to define in detail the facile photochemistry exhibited by deazaflavins.